The change in the focal length of a zoom lens is accomplished by an internal change of magnifications of the lens groups which move during zooming. A conventional zoom lens used in thirty-five millimeter photography for single lens reflex cameras comprises four groups where there is a first fixed or focusing group, a second variator group which provides the great majority of change of magnification of the lens, a third compensator group which maintains the image in focus, and a fourth relay or objective lens. In most of these zoom lenses, the focal length ratio over the zoom range is approximately three to one or less, and only the variator moving group is necessary since aberrations are usually stable for magnifications of about 1/.sqroot.3 to .sqroot.3. The compensator group provides little, if any, of the magnification change. To increase the zooming range, it is necessary to provide an additional source of magnification change, usually an additional motion. One lens group motion can be extended to a magnification change of greater than three to one, but only at the expense of increased size.
Many zoom lenses having more than one "zooming" group have been proposed, but to achieve compactness in both length and diameter, such lenses usually include movement of the front or first group for zooming. If, however, for other system considerations, it is undesirable to move the first group, then the two motions should occupy the same space for minimum size; that is, while one zooming group is moving to change magnification, the other is providing image plane compensation, and then the first moving group compensates while the second provides the primary change of magnification. Such system is capable of a large focal length range within a minimum size, and is limited only by the fact that two variating groups can become a focal as a unit in the mid-range position, and would not necessarily provide for a constant image position for the entire focal length range.
Accordingly, the present invention provides a very compact zoom lens with a large zoom range and the front vertex distance remains constant over the zoom range. That is, the front group does not move for zooming, but may move for focusing.